The present invention relates to a resin-molded semiconductor device in which semiconductor chip, leadframe and so on are molded with a resin encapsulant. In particular, the present invention relates to an improved device with the backside of a die pad exposed to radiate heat from a built-in power electronic device more efficiently.
In recent years, to catch up with rapidly advancing downsizing of electronic units, it has become increasingly necessary to assemble semiconductor components for those electronic units at a higher and higher density. Correspondingly, sizes and thicknesses of the semiconductor components such as resin-molded semiconductor devices, in which semiconductor chip, leadframe and so on are molded with a resin encapsulant, have also been noticeably reduced. Examples of resin-molded semiconductor devices accomplishing these objects include a so-called xe2x80x9cquad flat non-leaded (QFN)xe2x80x9d package. From the QFN package, outer leads, which are usually provided to protrude laterally out of a package, are eliminated. Instead, external electrodes to be electrically connected to a motherboard are provided on the backside of the QFN package.
Particularly when a power electronic device is built in a semiconductor chip, the resin-molded semiconductor device should have its size or thickness reduced while taking its heat radiation properties into account. Thus, a QFN package for a power electronic device (hereinafter, simply referred to as a xe2x80x9cpower QFN packagexe2x80x9d) has intentionally exposed the backside of a die pad, on which a semiconductor chip is mounted, not covered with a resin encapsulant. Hereinafter, structure and manufacturing method of a conventional power QFN package will be described.
FIG. 18(a) is a perspective view of a conventional power QFN package; FIG. 18(b) is a cross-sectional view thereof taken along the line XVIIIbxe2x80x94XVIIIb in FIG. 18(a); and FIG. 18(c) is a bottom view thereof.
As shown in FIGS. 18(a) through 18(c), the conventional power QFN package includes a leadframe consisting of: signal leads 101; a die pad 102; and support leads 103 for supporting the die pad 102. A semiconductor chip 104 with a built-in power electronic device is bonded on the die pad 102 with an adhesive 108, and electrode pads (not shown) of the chip 104 are electrically connected to the signal leads 101 with metal fine wires 105. And the die pad 102 except for its backside, semiconductor chip 104, signal leads 101, support leads 103 and metal fine wires 105 are molded with a resin encapsulant 106. In this structure, no resin encapsulant 106 exists on the backside of the signal leads 101. In other words, the backside of the signal leads 101 is exposed and the respective lower parts of the signal leads 101, including the exposed back surfaces thereof, are used as external electrodes 101a. 
The backside 102a of the die pad 102 is not covered with the resin encapsulant 106 either, but functions as an exposed heat-radiating plate. By bringing this die pad 102 into contact with a heat-radiating portion of a motherboard, a heat quantity, which has been emitted from the power electronic device consuming a lot of power, can be dissipated, thus suppressing a rise in temperature within the package.
According to the conventional technique, when the power QFN package is mounted on a motherboard such as a printed wiring board, solder ball electrodes are attached onto the external electrodes 101a to ensure a standoff height as measured from the backside of the resin encapsulant 106. This is done because the standoff height is required in bonding the external electrodes 101a, i.e., the lower parts of the signal leads 101, to the electrodes of the motherboard. After the standoff height has been ensured by providing these ball electrodes in this manner, the package is mounted on the motherboard.
A power QFN package like this may be manufactured by performing the following process steps, for example. First, a leadframe, including the signal leads 101, die pad 102, support leads 103 and so on, is prepared. It should be noted that the leadframe prepared is often provided with dam bars for preventing the overflow of a resin encapsulant during resin molding. Next, the semiconductor chip 104 is bonded, with the adhesive 108, onto the die pad 102 of the leadframe prepared. This process step is called xe2x80x9cdie bondingxe2x80x9d. Then, the semiconductor chip 104, which has been bonded onto the die pad 102, is electrically connected to the signal leads 101 with the metal fine wires 105. This process step is called xe2x80x9cwire bondingxe2x80x9d. As the metal fine wires 105, aluminum (Al) or gold (Au) fine wires may be appropriately used, for example.
Subsequently, the semiconductor chip 104, part of the die pad 102 except for the backside thereof, signal leads 101, support leads 103 and metal fine wires 105 are molded with the resin encapsulant 106 such as an epoxy resin. In this case, the leadframe, on which the semiconductor chip 104 has been bonded, is introduced into a molding die assembly and transfer-molded. In particular, resin molding is performed with the backside of the signal leads 101 in contact with the upper or lower die of the die assembly. Finally, the ends of the signal leads 101, protruding outward from the resin encapsulant 106, are cut off after the resin molding process step. By performing this cutting process step, the end faces of the signal leads 101 cut off are substantially flush with the side faces of the resin encapsulant 106. That is to say, this structure does not include any outer leads, which are ordinarily provided as external terminals. Instead, solder ball electrodes are provided for this structure as alternative external terminals under the external electrodes 101a, which are respective exposed lower parts of the signal leads 101 not covered with the resin encapsulant 106. As the case may be, a solder plating layer is sometimes formed in place of the solder balls.
The conventional power QFN package, however, has the following drawbacks. First, since the lower surfaces of the external electrodes 101a are located in substantially the same plane as the resin encapsulant 106 on the backside of the semiconductor device, no standoff height is ensured as measured from the bottom of the resin encapsulant 106. Thus, the device must be mounted onto a motherboard with the solder ball electrodes interposed therebetween. Accordingly, mounting cannot be carried out efficiently.
In the conventional manufacturing process of a resin-molded semiconductor device, a leadframe, on which a semiconductor chip has been bonded, is introduced into a die assembly. Then, the leadframe with the chip mounted is molded with a resin by pressing the signal leads against the surface of the lower die such that the leads come into close contact with the die. Even so, there occurs a problem that the resin encapsulant reaches the backside of the signal leads to form a resin burr (overflowed resin) on the surface of the external electrodes.
Thus, according to a proposed technique, a seal tape is interposed between the lower surface of the outer rail or the signal leads and the surface of the die assembly and resin molding is carried out with the lower parts of the signal leads forced into the seal tape. In this manner, those lower parts of the signal leads are protruded downward out of the resin encapsulant. In such a case, however, if the outer rail is deformed due to the clamping force applied to the outer rail and to the signal leads neighboring the outer rail, the force causing that deformation might be transmitted to the die pad by way of the support leads. As a result, the die pad might be deformed or displaced. It is imaginable to eliminate the support leads to obviate such inconvenience. Nevertheless, the reliability of the package might be risked because the die pad could not be supported with certainty in such a case.
In view of these respects, part of a support lead is preferably bent to form a raised portion higher in level than the other portions of the support lead. Then, the support lead can function as a sort of spring cushioning the deformation of the die pad. Accordingly, it is probably possible to prevent the die pad from being deformed due to the clamping force applied to the outer rail of the leadframe.
If the support leads are expected to perform a deformation cushioning function by providing those raised portions therefor, however, such a structure lacks in adaptability to chips of various sizes. Specifically, aside from semiconductor chips of relatively small sizes, if semiconductor chips of relatively large sizes are mounted on such a leadframe structure, those chips might be hampered by the raised portions of the signal leads.
An object of the present invention is providing a resin-molded semiconductor device adaptable to semiconductor chips of widely varying sizes and a method for manufacturing the same.
Another object of the present invention is improving the reliability of a resin-molded semiconductor device by preventing a resin encapsulant from being peeled off a die pad with a lot more certainty.
A leadframe according to the present invention is used for manufacturing a resin-molded semiconductor device. The leadframe includes: an outer rail surrounding an opening, in which a semiconductor chip will be mounted; a die pad provided inside the opening; a plurality of support leads for supporting the die pad; and a plurality of signal leads, which are connected to the outer rail around the periphery of the opening and extend toward the die pad. Each said support lead is provided with a raised intermediate portion higher in level than the other portion thereof. A central portion of the die pad is elevated above a peripheral portion thereof surrounding the central portion to support the semiconductor chip thereon.
In the leadframe according to the present invention, each support lead is provided with a raised portion and can cushion the force causing deformation. Thus, when resin molding is performed using this leadframe and a seal tape Such that a lower part of each signal lead protrudes out of the resin encapsulant, the support leads are not deformed even with the clamping force applied to the outer rail of the leadframe. Accordingly, it is possible to prevent the die pad from being deformed or displaced because of the clamping force. In addition, since the central portion of the die pad is elevated above the peripheral portion thereof, even a semiconductor chip of a relatively large size is not hampered by the support leads. That is to say, the semiconductor chip mounted on the leadframe can be selected from a much broader size range. Moreover, since the semiconductor chip is mounted only on the central portion of the die pad, the resin encapsulant also exists between the peripheral portion of the die pad and the semiconductor chip. As a result, the semiconductor chip can be held by the resin encapsulant more strongly and a resin-molded semiconductor device with improved moisture resistance can be provided.
In one embodiment of the present invention, the central portion of the die pad is preferably elevated above the peripheral portion via a half-blanked portion. In such an embodiment, the central portion can be elevated with substantially no strain applied to the die pad.
In another embodiment of the present invention, the upper surface of the die pad at the central portion is preferably higher in level than the uppermost surface of the support leads at the raised portion. In such an embodiment, hampering between the semiconductor chip mounted and the support leads can be avoided easily and with more certainty.
In still another embodiment, the die pad is preferably partially punched between the central and peripheral portions thereof. In such an embodiment, when resin molding is carried out using this leadframe, the resin encapsulant can be poured down through the punched portions around the central portion. As a result, a resin-molded semiconductor device, in which the central portion of the die pad is in much closer contact with the resin encapsulant, can be provided.
A first exemplary resin-molded semiconductor device according to the present invention includes: a die pad, a central portion of the die pad being elevated above a peripheral portion thereof surrounding the central portion; a semiconductor chip mounted on the central portion of the die pad; a plurality of support leads for supporting the die pad; a plurality of signal leads extending toward the die pad; a plurality of metal fine wires for electrically connecting the semiconductor chip to the signal leads; and a resin encapsulant for encapsulating the semiconductor chip, the die pad, the support leads, the metal fine wires and the signal leads such that the lower and outer side faces of each said signal lead are exposed as an external terminal and that a lower part of the signal lead protrudes downward. Each said support lead extends from the die pad to reach an associated side face of the resin encapsulant and is provided with a raised intermediate portion higher in level than the other portion of the support lead.
The first resin-molded semiconductor device performs the same functions and attains the same effects as those of the inventive leadframe. As a result, the size of a semiconductor chip to be housed in the resin-molded semiconductor device can be selected from a wider range and the moisture resistance of the device can be improved.
In one embodiment of the present invention, the lower surface of the semiconductor chip is preferably higher in level than the uppermost surface of each said support lead.
In another embodiment of the present invention, the raised portion of each said support lead is preferably sloped in such a manner as to reduce its height from a portion closest to the die pad toward the associated side face of the resin encapsulant.
In still another embodiment, the die pad is preferably partially punched between the central and peripheral portions thereof, and a region under the central portion of the die pad is preferably also filled in with the resin encapsulant. In such an embodiment, the central portion of the die pad can be in even closer contact with the resin encapsulant.
In still another embodiment, a closed-loop groove is preferably formed in the lower surface of the die pad at the peripheral portion. In such an embodiment, the resin encapsulant does not overflow onto the lower surface of the die pad at the peripheral portion.
A second exemplary resin-molded semiconductor device according to the present invention includes: a die pad, a central portion of the die pad being elevated above a peripheral portion thereof surrounding the central portion; a semiconductor chip mounted on the central portion of the die pad; a plurality of support leads for supporting the die pad; a plurality of signal leads extending toward the die pad; a plurality of metal fine wires for electrically connecting the semiconductor chip to the signal leads; and a resin encapsulant for encapsulating the semiconductor chip, the die pad, the support leads, the metal fine wires and the signal leads such that the lower and outer side faces of each said signal lead are exposed as an external terminal and that a lower part of the signal lead protrudes downward. Each said support lead extends from the die pad to reach an associated side face of the resin encapsulant and is provided with a raised intermediate portion higher in level than the other portion of the support lead. The semiconductor chip is supported by the raised portions of the support leads.
The second resin-molded semiconductor device also exhibits improved moisture resistance just like the first resin-molded semiconductor device. In addition, the semiconductor chip can be supported more stably by the support leads in the second device.
In one embodiment of the present invention, a closed-loop groove is also preferably formed in the lower surface of the die pad at the peripheral portion.
A third exemplary resin-molded semiconductor device according to the present invention includes: a die pad, a central portion of the die pad being elevated above a peripheral portion thereof surrounding the central portion; a semiconductor chip mounted on the central portion of the die pad; a plurality of support leads for supporting the die pad; a plurality of signal leads extending toward the die pad; a plurality of metal fine wires for electrically connecting the semiconductor chip to the signal leads; and a resin encapsulant for encapsulating the semiconductor chip, the die pad, the support leads, the metal fine wires and the signal leads such that the lower and outer side faces of each said signal lead are exposed as an external terminal and that a lower part of the signal lead protrudes downward. Part of the resin encapsulant is interposed between the upper surface of the die pad at the peripheral portion and the backside of the semiconductor chip. A groove is provided in the upper surface of the die pad at the peripheral portion so as to surround the central portion.
In the resin-molded semiconductor device according to any embodiment of the present invention, a gap exists between the upper surface of the die pad at the peripheral portion surrounding the central portion thereof and the backside of the semiconductor chip. By filling in the gap with the resin encapsulant, the resin encapsulant can be in closer contact with the upper surface of the peripheral portion. If the moisture resistance of the device deteriorates or if a thermal stress is caused in the device, part of the resin encapsulant filling the gap between the peripheral portion of the die pad and the semiconductor chip might peel off the die pad over a wider and wider area. In contrast, the third resin-molded semiconductor device is provided with a groove in the upper surface of the peripheral portion, where peeling of the resin encapsulant possibly happens. Accordingly, even if the resin is peeling off the die pad over an increasingly wide area, that peeled portion can be trapped at the groove. Thus, the reliability of the resin-molded semiconductor device can be maintained by preventing that peeling of the resin off the die pad from expanding.
In one embodiment of the present invention, the central portion is preferably elevated by pressworking and half-blanking the die pad, and preferably has a substantially circular planar shape.
In another embodiment of the present invention, a plurality of the grooves are preferably provided.
In still another embodiment, each said support lead preferably extends from the die pad to reach an associated side face of the resin encapsulant and is preferably provided with a raised intermediate portion higher in level than the other portion of the support lead.
A first exemplary method for manufacturing a resin-molded semiconductor device according to the present invention includes the steps of a) preparing a semiconductor chip and b) preparing a leadframe. The leadframe includes: an outer rail surrounding an opening, in which the semiconductor chip will be mounted; a die pad provided inside the opening, a central portion of the die pad being elevated above a peripheral portion thereof surrounding the central portion; a plurality of support leads for connecting the die pad to the outer rail, each said support lead including a raised intermediate portion higher in level than the other portion thereof; and a plurality of signal leads, which are connected to the outer rail around the periphery of the opening and extend toward the die pad. The method further includes the steps of: c) bonding the semiconductor chip on the central portion of the die pad in the leadframe; d) electrically connecting the semiconductor chip to the signal leads via connection members; and e) molding the semiconductor chip, the die pad, the connection members, the support leads and the signal leads with a resin encapsulant with a seal tape interposed between the backside of the leadframe and a die assembly and with clamping force applied to the leadframe and the seal tape.
According to the first method, the size restrictions of semiconductor chips can be eased, the deformation or displacement of the die pad during resin molding can be suppressed and the lower part of each signal lead can be protruded out of the resin encapsulant.
In one embodiment of the present invention, the clamping force is preferably applied to the outer rail of the leadframe and to the seal tape in the step e). In such an embodiment, the lower part of each signal lead adjacent to the outer rail can have its protrusion height increased.
In another embodiment of the present invention, a closed-loop groove is preferably formed in the lower surface of the die pad at the peripheral portion in the step b). In such an embodiment, it is possible to prevent the resin encapsulant from overflowing so far as to reach the lower surface of the die pad at the peripheral portion.
In still another embodiment, the die pad is preferably partially punched between the central and peripheral portions thereof in the step b). In such an embodiment, the resin encapsulant can be poured down through the punched portions around the central portion of the die pad during the resin molding step e).
A second exemplary method for manufacturing a resin-molded semiconductor device according to the present invention includes the steps of a) preparing a semiconductor chip with electrodes and b) preparing a leadframe. The leadframe includes: an outer rail surrounding an opening, in which the semiconductor chip will be mounted; a die pad provided inside the opening, a central portion of the die pad being elevated above a peripheral portion thereof surrounding the central portion, a groove being provided in the upper surface of the die pad at the peripheral portion; a plurality of support leads for supporting the die pad; and a plurality of signal leads, one end of each said signal lead being connected to the outer rail, while the other end thereof extending toward the die pad. The method further includes the steps of: c) securing the semiconductor chip onto the die pad by bonding the upper surface of the die pad at the central portion to the backside of the semiconductor chip via an adhesive; d) electrically connecting the electrodes of the semiconductor chip mounted on the die pad to the signal leads of the leadframe with metal fine wires; e) molding the semiconductor chip, the die pad, the support leads, the metal fine wires and the signal leads with a resin encapsulant such that the lower and outer side faces of each said signal lead are exposed as an external terminal and that a lower part of the signal lead protrudes downward; and f) partially cutting the signal leads off such that an end face of each said signal lead is substantially flush with an associated side face of the resin encapsulant, and cutting the support leads off to remove the resin-molded semiconductor device from the outer rail of the leadframe.
According to the second method, a highly reliable resin-molded semiconductor device can be obtained by stopping the progress of peeling of the resin off the die pad.
In one embodiment of the present invention, the central portion of the die pad is preferably elevated above the peripheral portion by pressworking and half-blanking the die pad in the step b). In such an embodiment, strain, which might be caused in the die pad when the central portion thereof is elevated above the peripheral portion, can be minimized.